In 21 years, Kevin will be 4 times as old as he is right now. How old is he right now?
Answer: We can use the given information to write down an equation about Kevin's age. Let Kevin's age be $k$ In 21 years, he will be $k + 21$ years old. At that time, he will also be $4 k$ years old. Writing this information as an equation, we get: $k + 21 = 4 k$ Solving for $k$ , we get: $3 k = 21$ $k = 7$.